[Mobile Perf Recipe] Add the benchmarking part for iOS

Author: xta0

recipes_source/mobile_perf.rst

@@ -221,3 +221,45 @@ Now we are ready to benchmark your model:
   Main run finished. Microseconds per iter: 121318. Iters per second: 8.24281
 
 
+iOS
+-------
+
+For iOS, we'll be using our `TestApp <https://github.com/pytorch/pytorch/tree/master/ios/TestApp>_` as the benchmarking tool. 
+
+To begin with, let's apply the ``optimize_for_mobile`` method to our python script located at `TestApp/benchmark/trace_mode.py <https://github.com/pytorch/pytorch/blob/master/ios/TestApp/benchmark/trace_model.py>`_. Simply modify the code as below.
+
+::
+
+  import torch
+  import torchvision
+  from torch.utils.mobile_optimizer import optimize_for_mobile
+
+  model = torchvision.models.mobilenet_v2(pretrained=True)
+  model.eval()
+  example = torch.rand(1, 3, 224, 224)
+  traced_script_module = torch.jit.trace(model, example)
+  torchscript_model_optimized = optimize_for_mobile(traced_script_module)
+  torch.jit.save(torchscript_model_optimized, "model.pt")
+
+Now let's run ``python trace_model.py``. If everything works well, we should be able to generate our optimized model in the benchmark directory. 
+
+Next, we're going to build the PyTorch libraries from source.
+
+::
+
+  BUILD_PYTORCH_MOBILE=1 IOS_ARCH=arm64 ./scripts/build_ios.sh
+
+Now that we have the optimized model and PyTorch ready, it's time to generate our XCode project and do benchmarking. To do that, we'll be using a ruby script - `setup.rb` which does the heavy lifting jobs of setting up the XCode project. 
+
+::
+
+  ruby setup.rb
+
+Now open the `TestApp.xcodeproj` and plug in your iPhone, you're ready to go. Below is an example result from iPhoneX
+
+::
+
+  TestApp[2121:722447] Main runs
+  TestApp[2121:722447] Main run finished. Milliseconds per iter: 28.767
+  TestApp[2121:722447] Iters per second: : 34.762
+  TestApp[2121:722447] Done.